The SDL build reads vMac.ROM, fixed at build time to the Mac Plus ROM name and 128 KB (setup/SPCNFGAP.i:60-63), and checks it against three Mac Plus checksums (src/CONTROLM.h:1344-1388). Search order: --rom, the executable's directory, mnvm_rom/ under the SDL pref path, the working directory (src/OSGLUSDL.c:546-620).
